1. Field of the Invention
The present invention relates to a decision of drive timing for an engine driven generator which is mounted in an electric vehicle.
2. Description of the Related Arts
In recent years advanced investigation has been made of electric vehicles from a viewpoint of its lower polluting properties. However, electric vehicles entail a drawback that the distance capable of being traveled per unit charge depends on the size of the battery loaded thereinto. Thus, attention is being given to the electric vehicle having an engine driven generator mounted therein.
This electric vehicle containing the engine driven generator usually travels as an electric vehicle and, when the capacity of the battery is deficient, drives the engine to effect the generation of electricity. In other words, when the state of charge of the battery is lowered, the engine is started to effect the electricity generation, and at the completion of the charge of the battery, the generation of electricity comes to a stop. Due to application to the electricity generation, in this case, the engine is capable of being set at substantially constant rotational speed and constant load, thereby suppressing the exhaust of polluting substances to a lower level.
This type of electric vehicle having such an engine driven generator mounted therein is disclosed in Japanese Patent Laid-open Publication No. 55-157901.
In the electric vehicle having such a conventional engine driven generator mounted therein, there is a need for a sensor (SOC sensor) for detecting the state of charge (SOC) of the battery in order to control the activation and stop of the engine driven generator. The apparatus disclosed in the above publication uses a pycnometer for detecting a specific gravity of the battery liquid in order to detect the SOC.
Disadvantageously, this involves a drawback that the battery must be modified to mount the pycnometer. In addition, the battery of the electric vehicle is divided into a multiplicity of cells in view of outputting a high-voltage in the order of 200 V. Thus, the SOC obtained by the pycnometer may not accurately represent the SOC of the entire battery.
Some SOC sensors integrate the current discharged from the battery and, based on the amount of discharge, detect the SOC. These types of SOC sensors eliminate the necessity of modifying the battery itself. However, such SOC sensors require to be reset with a periodic full charge, and if charge and discharge are repeated for a long period of time without full charge, errors may accumulate. In particular, in the case where the charge is performed by the engine driven generator mounted in the vehicle, there is a large possibility of such a state occurring since the battery is not fully charged by this charge. Also, in the case of deterioration of the battery, the errors may be increased.
If the SOC is not accurately detected, there may occur an overcharge or overdischarge, which may deteriorate the battery.
It is conceivable that when the SOC of the battery is lowered, the voltage is lowered and hence the SOC may be detected based on the voltage values. However, because the battery voltage is varied in accordance with the amount of load current to be applied to the load, a mere detection of the battery voltage does not permit an accurate grasp of the SOC. Under the condition separated from the load, it is when the SOC is lowered considerably that the voltage is largely reduced.
On the contrary, it is known that the voltage value at a larger load current presents a fairly good correlation with the SOC of the battery. Therefore, in Japanese Patent Laid-open Publication No. 50-21210, the SOC is considered to be deteriorated if the battery voltage is not more than a predetermined value when the load current applied to the motor is a large current not less than a predetermined value, thereby controllably actuating the engine driven generator to Initiate the charge. According to this method, the SOC is detected based on the battery voltage, and the engine driven generator can be controlled without providing any specific SOC detector.
In the prior art described above, however, the detection of the SOC, that is, the timing to start the operation of the generator is limited to the case of larger load current. This results in a problem that the operation of the generator is not started under the conditions where the travel is continued with a relatively small load current and the battery is discharged. In the ordinary case, the capacity of the electricity to be generated by the generator is set on the basis of load current under ordinary travel which is not so large. Thus, if the generation of electricity is Initiated with a larger load current, subsequent SOC of the battery may be possibly lowered. Then, if the travel is stopped, the key is removed, and the vehicle function is stopped under the deteriorated conditions of the SOC of the battery, a large amount of current is not allowed to flow due to the reduction in the SOC at the time of restart. Therefore, the generator is not started, which makes the travel as an electric vehicle impossible. In this case, it may also be impossible to start the engine for the generator.